In recent
years, increasing numbers of radio hobbyists have
wished to attach multiple receivers to the same
antenna. This need may stem from a group wishing
to share a single antenna on a DXpedition to a remote
site, or it may be a single hobbyist wishing to
operate two or more receivers simultaneously.

In any
case, many of us have found out that simply using
a stub of wire to hook the antenna ports of two
or more receivers to the same antenna is an invitation
to all sorts of problems. One of the funnier problems
can occur if one of several receivers hooked together
presents significantly lower impedance to the antenna
than do the others. Years
ago, when Mitch Sams, Kirk Allen and John Bryant
first "shared" a beverage antenna, Kirk
and John spent a frustrating half-night wondering
why Mitch's old receiver was so much superior to
their more modern gear: they eventually realized
that Mitch's old receiver was literally sucking
up all of their DX!

A second
common occurrence when hooking multiple receivers
together is that spurious radiations/local oscillator
signals from one receiver can use the common antenna
lead as a pathway to enter the other receivers sharing
the antenna; this can cause serious but difficult
to recognize interference or partial loss of signal.
For all of these reasons and more, if you wish to
operate two or more receivers, simultaneously, you
will need to use a device called variously, an antenna
splitter, a signal splitter or a power splitter:
when referring to a receiving antenna device, most
people use these three terms interchangeably.

Most signal splitters are based
on a fundamental building block which is a transformer-like
device that accepts a single signal stream and splits
it into two identical parts that are each (by the
laws of physics) diminished in strength by about
3 dB, minimum. Usually, these transformer-like devices
consist of a ferrite core and windings of fine wire
and this building block may be diagrammed as an
upside-down capital letter "Y." Antenna
splitters that offer four output ports are usually
simply three "building blocks" arranged
in a cascade fashion, where the first unit splits
the signal into two halves, which are then fed into
a second rank of two splitters; those second rank
splitters divide the half signals into halves again,
creating four identical signals of further diminished
strength. Since each transformation/splitting incurs
about 3 dB of loss, it is easy to see why most splitters
of four output ports or more also include RF amplification.

The first antenna splitters that many of us saw
were rather expensive and complex devices built
with vacuum tube technology. These devices, usually
purchased used from government surplus property
outlets, often support 8 or 16 receivers simultaneously
and contained sophisticated RF amplifiers, as well.
In more recent times, smaller-scale solid-state
splitters have become available commercially, intended
for both the professional and serious hobbyist markets.
The most commonly available splitters are 2-port,
unamplified units. However, 4-port units, either
with or without internal amplification are also
currently available and two of the three splitter
manufacturers produce a bewildering array of splitters
suited for many professional communications uses.

To our knowledge
no technical comparisons of these rather costly
devices have been published. Since both of us were
interested in either purchasing splitters or building
them from scratch, we recently undertook to evaluate
what was available on the market. Happily, Bill
Bowers, a retired engineer, has an array of professional
test equipment and was interested in running the
tests. John Bryant served as cheerleader and scribe
of the project.

The Splitters

Ideally, we would all be
using antennas that have 50 ohms impedance, which
would couple to feed lines having 50 ohm impedance
that would, in turn, match the 50 ohm input impedance
of our receivers. We recently published a study
similar to this one which outlined tests of the
impedance transformers necessary to match several
forms of wire antennas (often from 200 to 2000 ohm
impedance according to type and size) to feed lines
that are nominally at 50 ohm impedance. All of the
splitters evaluated in this study are meant to be
used within 50-ohm antenna systems.

Since the internal circuitry of most large splitter
units are simply multiple two-port circuits which
are cascaded to make, 4, 8 or 16 port units, we
chose to concentrate our effort on three commercially
available two-port units. We also included one home-built
two port unit which was based on a design originally
received from Sam Dellitt in Australia that has
been published rather widely in the hobby press.
In the initial testing, Bill also included one passive
four-port splitter (Mini-Circuits Model ZSC-4-3B)
and one six-port splitter, Model MCL, which appears
to actually be a Mini-Circuits Model ZFSC-6-110.
These latter two were obtained in used condition
from an on-line auction.

Model MC-102 was purchased directly
from Stridsberg Engineering, 345 Albert Avenue Shreveport,
LA 71105 Telephone: (318) 861-0660, FAX: (318) 861-7068
(www.stridsberg.com). The current retail price (2004)
is $65 plus S&H. Frequency coverage, as listed
by the manufacturer is 100 kHz to 500 MHz.The Stridsberg
unit is exceptionally well finished and is the largest
of the three commercial splitters, measuring about
5"W x3"D x1.5"H, counting the ports.
The company welcomes telephone orders, for even
a single unit, and does ship overseas.

The RF Systems Model
SP-1 is available from several hobby sources around
the world. Our test unit was purchased for $89.95
plus S&H from our friends at Universal Radio
in Reynoldsberg, OH. They accept web orders at http://www.universal-radio.com/catalog/preamps.html
or sell to hobbyists by phone at 1-800-431-3939
(Orders & Prices Only) and also ship overseas.
RF Systems lists frequency coverage as 50 kHz to
30 MHz. As you will note from the photograph, the
SP-1 comes with SO-239 ports, so adapters were used
to fit the unit to Bill's BNC-equipped test gear.
The box itself is about 1" square and 4"
long. However, with adapters to BNC fittings, it
occupies about 4"x 4"x1" of space.

The Mini-Circuits
Model ZSC-2-2 was purchased directly from the Mini-Circuit
sales office in Missouri (phone: 718-934-4500, fax:
718-332-4661) for $52.95 plus shipping and handling.
Mini-Circuits lists frequency coverage as 2 kHz.
To 60 MHz. Despite being the largest of the three
organizations and being primarily committed to the
governmental and commercial markets, they specifically
welcome small orders from hobbyists. They do ship
overseas. They also provide excellent technical
information about their products, but no ordering
capability, at www.minicircuits.com The Mini-Circuits
splitter is the most compact of the three, measuring
2.25"W x 1.5"D x 1.6"D, including
the ports.

The Tests

The following characteristics
will be measured over a range of frequencies from
150 kHz. to 13 MHz.:

ANTENNA IMPEDANCE MATCH:
This is the impedance that will terminate the coax
lead in cable from the antenna. The RG-58 has a
characteristic impedance of approximately 50 Ohms
and if the antenna port of the splitter has an impedance
other than 50 Ohms, part of the signal will be reflected
back to the antenna. The greater the impedance of
the antenna port differs from 50 Ohms, the greater
will be the signal loss. The amount of loss is rather
complex and the total loss also depends on the length
and attenuation of the coax. This impedance was
measured at the antenna port with all receiver ports
terminated in 50 ohms, resistive. The ideal splitter
would present 50 Ohms at the antenna port.

RECEIVER IMPEDANCE MATCH:
This impedance, in an ideal splitter, should also
be 50 Ohms to match the 50 Ohm impedance of the
receiver antenna terminal. The mismatch here is
not quite as important as there is usually a very
short cable between the splitter and the receiver.
Further, The 50 ohm input impedance of the receiver
is often fairly well defined over a certain bandwidth.
This measurement was made at a receiver port when
the other receiver port and the antenna port terminated
in 50 Ohms, resistive.Table
2 (MS Word format) on Receiver Impedance Match

SIGNAL ATTENUATION:
The attenuation of a signal, from a 50 Ohm source,
as it passes from the antenna port to one of the
splitter receiver ports. The other receiver port(s)
are terminated in 50 Ohms, resistive. The attenuation
of a signal, when it is split 2 ways, in an ideal
splitter would be 3db, when split 4 ways is 6 db,
etc.Table
3 (MS Word format) on Signal Attenuation

SIGNAL
ISOLATION: The local oscillator of a receiver
radiates back out the antenna connection and thus
into the splitter. To prevent one receiver's oscillator
from interfering with the signal going into the
other receiver connected to the splitter, it is
desirable to have as much signal isolation as possible.
The larger the signal isolation the better. For
this test, the attenuation, from a 50 Ohm source
connected to a receiver port, was measured at another
receiver port. All receiver ports and the antenna
port were terminated in 50 Ohms, resistive.Table
4 (MS Word format) on Signal Isolation

IMPEDANCE ISOLATION:
The antenna input impedance of a receiver with a
"coax connection" is nominally 50 ohms
when it is tuned to the incoming signal. Some receivers
show an impedance as low as 10 ohms at frequencies
other than the one to which the receiver is tuned.
This 10 Ohm load at one receiver port of the splitter
can upset the impedance seen at the other port.
Here again the ideal splitter would continue to
present 50 Ohms impedance even when the other port
is loaded with 10 Ohms. This measurement was made
at one receiver port as the impedance at one of
the other receiver ports was reduced from 50 to
10 Ohms, resistive.

TOTAL
LOSS: This is the total loss of signal as
it passes from the antenna through 100 feet of RG-58A/U
coax and a signal splitter. A measured signal, (-20dbm.)
is fed into the coax at the antenna end and the
strength of the signal at the receiver end was measured
with a Fluke 8922A RF voltmeter. This is a comparison
of the quality of the splitters under typical conditions.

One area of
some concern was noted in the Signal Isolation tests:
the relatively poor performance of the Stridsberg
and RF Systems units at the lower end of medium
and long wave frequencies. While this would be of
little concern to shortwave DXers and most radio
amateurs, people with interests in the regions at
and below 1 MHz should take note. We were both also
surprised at the impedance mismatches exhibited
by both the Stridsberg MC-102 and RF Systems SP-1
in the tests of Antenna Impedance Match, Receiver
Impedance Match and Impedance Isolation. In some
cases, these mismatches reached 100%. However, the
measured signal losses of all three units are very
nearly equal, reminding us again how forgiving receiving-only
devices are of mismatches.

The last test, Total Loss, is really the bottom
line. It indicates that, for most uses, there is
really no significant difference between the three
commercial products and that a selection could be
made based on price and availability. However, for
the "extreme DXer" who may be unwilling
to give up even half a decibel of signal, or for
some technical applications, the Mini-Circuits ZSC-2-2
was clearly the best in each measured characteristic.
The home-brew splitter was the second best in most
tests below 5 MHz. Its performance encourages us
to undertake a second project in the very near future
to further develop the homebrew design.

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